Putting a cup with water in the incubator will reduce surface area and it will take much longer time for the water to evaporate. It will be better to have a pan of some type because they are usually wider and more water will fit. Some small incubators that hatch about 4-10 eggs or so do require 1 cup of water. Putting a pan with water is much better.
It all depends of what size of an incubator you have. There are some incubators that hold up to 40 eggs, others 216 chickens eggs. The bigger the incubator, the more water it requires.